The coding and decoding of the interleaved amplitude samples may be performed by a single coder and a single decoder common to all the channels served by the respective terminal, but this may give rise to objectionable crosstalk; moreover, such a common coder or decoder must operate at a very high speed commensurate with the number of bits in all the interleaved code words. Thus, the use of individual coders and decoders for the several channels is often preferred.
Conventional coders work with a ramp-function generator producing a linearly varying reference voltage whose instantaneous magnitude is compared with the signal amplitude to be encoded; the time required for the reference voltage to reach the signal amplitude is a measure of the value of the latter and can be translated into binary form by determining the number of counting pulses in a sequence of such pulses starting at the beginning of each coding interval. In order to limit the range of signal amplitudes to be encoded, it is desirable to operate on a generally exponential compression characteristic during coding and on a complementary expansion characteristic during decoding. This companding effect can be achieved by progressively decreasing the cadence of the counting pulses during a coding interval (and correspondingly increasing that cadence on decoding) so that the counting rate varies with the slope of the desired characteristic.
In such a companding system, however, pulses must be generated and counted during part of a cycle at a rate which is a multiple of the mean counting rate, i.e. the pulse-repetition frequency that would be required for linear coding. In the case of a bipolar characteristic whose positive and negative branches are each divided into eight straight segments approximating an exponential curve (see for example U.S. Pat. No. 3,688,221), the slope of the first two segments is 32 times as high as that of the final segment so that the initial counting speed must be 16 times the mean. With a sampling rate of, say, 8 kHz and 2.sup.7 =128 quantizing levels for the signal amplitude, semiconductors adapted to handle such high pulse cadences are quite costly.